Growth of Pinus sylvestris 6-11 years after inverted site preparation : comparison of inverted site preparation and disc trenching’s effect on tree growth and pre-commercial thinning needs

Sammanfattning

Mechanical site preparation (MSP) is widely used to improve microsite conditions for forest regeneration, with disc trenching being the most utilized method in Swedish forestry. Inverted site preparation has been proposed as an alternative MSP method, as previous studies have shown reduced disturbance of ground vegetation while maintaining or enhancing seedling growth. However, the effects of inverted site preparation have primarily been evaluated under controlled experimental conditions, and its performance under operational forestry practises remains insufficiently documented. This thesis evaluates regeneration outcomes following inverted site preparation carried out using the continuously advancing towed unit Kicken, in comparison with disc trenching. Regeneration was inventoried at 30 operational forest sites regenerated with Scots pine (Pinus sylvestris L.) in Västerbotten County, with site age ranging from 6-11 years. The study included 15 sites prepared using inverted site preparation and 15 sites prepared using disc-trenching, which were selected based on comparable site characteristics observed in the field. Seedling growth and natural regeneration were inventoried using 10 systematically placed sampling plots per site. Statistical analyses were conducted using a two-way ANOVA, with site preparation method and site index (SI) included as fixed factors. No statistically significant differences were detected in seedling height, diameter or leader shoot length between site preparation methods, and similar results were obtained for mean annual height and diameter growth. In contrast, disc-trenched sites exhibited significantly higher densities of naturally regenerated birch (Betula pubescens Ehrh. and Betula pendula Roth), while total natural regeneration did not differ significantly between methods, although mean regeneration densities were higher following disc trenching. These results suggest that inverted site preparation using Kicken provides regeneration outcomes comparable to disc trenching under operational conditions, while potentially reducing the need for pre-commercial thinning due to lower birch regeneration.